Berkeley Lab scientists have created insulated electrical wires that
are about 100,000 times narrower in diameter than a human hair. These
insulated wires are single-walled carbon nanotubes encased within an outer
sheath of boron nitride nanotubes. The ultra-high-strength wires were
reported in the April 18, 2003, issue of the journal Science.

"The ability to insulate nanowires opens up new possibilities for
nanoelectronics," says Alex Zettl, a physicist with Berkeley Lab's
Materials Sciences Division (MSD) and UC Berkeley's Physics Department,
who led the research. "Insulation keeps different wires from shorting
to each other or to nearby conductors, and will allow the wires to serve
as the basis of coaxial cables or a simple gating configuration for the
production of electronic devices such as transistors."

Coauthoring the Science paper with Zettl were William Mickelson, Shaul
Aloni, Wei-Qiang Han and John Cumings.

Nanoscience marches on

The inside diameter of a boron nitride
tube determines the configuration of the carbon buckyballs packed
inside it: (a) in a 2-nanometer tube, the buckyballs are staggered;
(b) in a 2.8-nanometer tube, the buckyballs assemble as rotating triangles;
(c) at a diameter of 3.3 nanometers, the buckyball nanowire is shaped
like a corkscrew

First came the discovery in 1985 of fullerenes, the cage like structures
of carbon atoms, the most famous of which is carbon-60, the buckyball.
Then came the discovery in 1991 of the long, thin, hollow cylinders of
carbon atoms called nanotubes. In 1998, scientists created the first carbon
"peapods," carbon nanotubes that were packed with a linear chain
of buckyballs.

Now, from Zettl's group, which in 1997 made the first nanotubes out of
pure boron nitride, comes the "boron nitride silo." When a boron
nitride silo is packed with buckyballs, then subjected to a ten-minute
blast from an intense beam of electrons, the result is a carbon nanowire
conductor enclosed within the world's strongest wide gap insulating fiber.

Zettl and his group made their boron nitride nanotubes using a plasma
arc technique, developed earlier in Zettl's laboratory, in which a hot
electrical discharge is sent between two boron-rich electrodes in a chamber
filled with pure nitrogen gas. This yields an abundance of boron nitride
nanotubes in the soot that forms along the chamber walls.

The soot can then be heat-treated to open the tips of the tubes, creating
a boron nitride silo. The silos were packed with buckyballs by sealing
the soot in vacuum inside a quartz ampoule along with carbon-60 powder,
then heating the ampoule to between 550 and 630 degrees Celsius for 24
to 48 hours.

A slew of possibilities

In addition to creating insulated carbon nanowires, Zettl says that
boron nitride silos can serve as model systems for studying the mechanical,
electronic, thermal, and magnetic properties of "dimensionally-constrained"
configurations of densely packed molecules such as buckyballs, a critical
need for the development of nanotechnology.

"When individual carbon-60 spheres just barely fit inside the boron
nitride cylinder, we got the linear-chain or classic peapod configuration,"
Zettl says. "However, at a slightly larger cylinder diameter, the
spheres began to form a staggered chain. At still larger diameters, the
staggered chain became close-packed, followed by a corkscrew-like formation."